1. Field of the Invention
The present invention relates generally to a resistance welding apparatus using an inverter, and more particularly to a resistance welding control apparatus for the execution of inverter-controlled welding current supply.
2. Description of the Related Arts
FIG. 12 shows a conventional resistance welding apparatus using an inverter.
In this resistance welding apparatus, the inverter designated at 104 is electrically connected between a rectifying circuit 100 and a primary coil of a welding transformer 106.
The rectifying circuit 100 rectifies a three-phase AC voltage at a commercial frequency from AC power supply terminals, e.g., three-phase AC power supply terminals (U, V, W), into a DC voltage. The DC voltage output from the rectifying circuit 100 is fed via a capacitor 102 to input terminals (l.sub.0, l.sub.1) of the inverter 104.
The inverter 104 consists of four uni-directional conduction switching elements q.sub.1, q.sub.2, q.sub.3 and q.sub.4 typically in the form of transistors, which are bridge connected to each other. Diodes d.sub.1, d.sub.2, d.sub.3 and d.sub.4 are connected in parallel to the switching elements q.sub.1, q.sub.2, q.sub.3 and q.sub.4, respectively, with their polarities being opposite to those of the switching elements.
Of the four switching elements, the switching elements q.sub.1 and q.sub.2 constituting a first (positive side) pair are turned ON/OFF at a time by a common switching control signal s.sub.a from a drive circuit 112, whereas the switching elements q.sub.3 and q.sub.4 constituting a second (negative side) pair are turned ON/OFF at a time by a common switching control signal s.sub.b from the drive circuit 112.
Output terminals (m.sub.0, m.sub.1) of the inverter 104 are connected to both ends of the primary coil of the welding transformer 106. A pair of welding electrodes 108 and 110 are electrically connected to both ends of a secondary coil of the welding transformer 106. The pair of electrodes 108 and 110 butt (in a confronting manner for example) against associated materials W.sub.1 and W.sub.2 to be welded and come into pressure contact with them under a pressing force from a pressure mechanism not shown.
A control unit 114 serves to control the supply of welding current, and during the weld time it provides a switching control of the inverter 104 by way of the drive circuit 112 as described above.
FIG. 13 shows a current supply control method effected in this resistance welding apparatus.
Typically, in the inverter resistance welding apparatus having the welding electrodes 108 and 110 directly connected to the secondary coil of the welding transformer 106 without any rectifying circuit intervening therebetween, as in the example shown, a half cycle is allocated to both a unit weld period T.sub.a during which a welding current (primary current) I.sub.2 flows continuously through the materials (W.sub.1, W.sub.2) to be welded in the positive direction and to a unit weld period T.sub.b during which it flows continuously therethrough in the negative direction, and the entire weld time from the start to the end of the welding current supply is set to be equal to the cycle count integer times the half cycle.
The unit weld periods T.sub.a and T.sub.b could be set to any length containing the unit cycle of the inverter frequency multiplied by positive integer, but it may be set to a period equal to the half cycle of 50 Hz or 60 Hz so as to correspond to the commercial frequency for example.
During the unit weld period T.sub.a, by way of the drive circuit 112 the control unit 114 turns ON/OFF the switching elements q.sub.1 and q.sub.2 on the positive side simultaneously at a predetermined inverter frequency, e.g., 10 kHz while the switching elements q.sub.3 and q.sub.4 on the negative side remain kept OFF.
Thus, at the output terminals (m.sub.0, m.sub.1) of the inverter 104 there appears a DC pulse-like primary current I.sub.1. which could otherwise be acquired by chopping the positive DC voltage by the period of the inverter frequency, the chopped current being fed to the primary coil of the welding transformer 106. Through the secondary circuit of the welding transformer 106 there flows in the positive direction a secondary current I.sub.2 having a current value at a predetermined ratio relative to the primary current I.sub.1, the current I.sub.2 having a current waveform obtained by smoothing the primary current I.sub.1.
During the unit weld period T.sub.b, by way of the drive circuit 112 the control unit 114 turns ON/OFF the switching elements q.sub.3 and q.sub.4 on the negative side simultaneously at the above inverter frequency while the switching elements q.sub.1 and q.sub.2 on the positive side remain OFF.
In consequence, at the output terminals (m.sub.0, m.sub.1) of the inverter 104 there appears a DC pulse-like primary current I.sub.1, which could otherwise be acquired by chopping the negative DC voltage by the period of the inverter frequency, the chopped current being fed to the primary coil of the welding transformer 106. Through the secondary circuit of the welding transformer 106 there flows in the negative direction a secondary current I.sub.2 having a current value at a predetermined ratio relative to the primary current I.sub.1, the current I.sub.2 having a current waveform obtained by smoothing the primary current I.sub.1.
It will be appreciated that a halt period Tc for the polarity switching is interposed between the end of each unit weld period and the start of the next unit weld period as shown in FIG. 13.
In the case of the current supply control method as described above, when the two switching elements q.sub.1 and q.sub.2 or q.sub.3 and q.sub.4 are changed over from ON state to OFF state in unit cycle T.sub.0 of the inverter frequency, the primary current I.sub.1 will not stop immediately due to the influence of inductance of the welding transformer 106, but it will flow as a transient current i through the primary circuit until it is shut off.
The transient current i upon the current interruption in the primary circuit will not pass through the switching elements q.sub.1, q.sub.2, q.sub.3 and q.sub.4, all of which are in OFF state, but it will flow through the diodes d.sub.1, d.sub.2, d.sub.3 and d.sub.4 instead, which are connected in parallel to the associated switching elements.
In the unit weld period T.sub.a for example, when both the switching elements q.sub.1 and q.sub.2 are changed over from ON state to OFF state in each unit cycle T.sub.0, the transient current i will make a closed circuit through which it flows from the primary coil of the welding transformer 106 via the diode d.sub.3, the capacitor 102 and the diode d.sub.4 again into the primary coil of the welding transformer 106, as indicated by a chain-dotted line of FIG. 12.
In this case, as indicated by hatched portions of FIG. 13, a positively directed flow is imparted to the transient current i (of the primary current I.sub.1) between the output terminals (m.sub.0, m.sub.1) of the inverter 104 and the primary coil of the welding transformer 106, whereas a negatively directed flow is imparted to the transient current i (of the primary current I.sub.0) between the input terminals (I.sub.0, I.sub.1) of the inverter 104 and the capacitor 106.
In the unit weld period T.sub.b on the other hand, when both the switching elements q.sub.3 and q.sub.4 are changed over from ON state to OFF state in each unit cycle T.sub.0, the transient current i although not shown will make a closed circuit through which it flows from the primary coil of the welding transformer 106 via the diode d.sub.1, the capacitor 102 and the diode d.sub.2 again into the primary coil of the welding transformer 106.
In this case, the transient current i (of the primary current I.sub.1) between the output terminals (m.sub.0, m.sub.1) of the inverter 104 and the primary coil of the welding transformer 106 will flow in the negative direction, and the transient current i (of the primary current I.sub.0) between the input terminals (I.sub.0, I.sub.1) of the inverter 104 and the capacitor 102 will also flow in the negative direction.
Such a conventional resistance welding apparatus entails a deficiency that the capacitor 102 may become worn out and therefore have a shortened lifetime due to heating, which will arise from the above transient current i flowing through the primary circuit of the welding transformer 106 immediately after the switching off of the inverter.
This capacitor 102 is provided intrinsically for smoothing the DC voltage from the rectifying circuit 100. At the same time, the capacitor is also one of the elements which form the closed circuit for allowing a flow of the transient current i. It has a relatively large capacitance and includes a resistance as well. The transient current i causes Joule heat to occur at this resistor. Accordingly as the amount of heating increases and the temperature rises, the capacitor 102 tends to become more worn out and degraded.
Incidentally, the transient current i is proportional to the welding current I.sub.2 and the primary current I.sub.1. For this reason, if a larger current flows through the materials to be welded, the transient current i will also become larger, resulting in accelerated wearout and degradation of the capacitor 102. As a result, inconveniently it may be difficult to select a large welding current if the lifetime of the capacitor 102 is taken into consideration.
Furthermore, the more frequent use or higher duty cycle the welding machine has, the larger the amount of heating of the capacitor 102 becomes due to the transient current i frequently flowing into the capacitor 102, consequently promoting its wearout and degradation. Therefore, from the viewpoint of lifetime of the capacitor 102, any restriction has to be offered to the duty cycle (rate of operation) as well.
Moreover, when the transient current i flows through the capacitor 102, Joule heat may be generated in conductors such as cables or copper bars as well, which electrically connect the capacitor 102 to the main circuit. This heating will cause the ambient temperature around the capacitor 102 to rise, resulting in shortened lifetime of the capacitor 102.
There may also arise another deficiency that because of vain consumption of power in the primary circuit due to heating of the capacitor 102 in this manner, corresponding reduction may occur in the amount of supply of power to the secondary side as well as in the power efficiency.